The present invention relates to measurement techniques. In particular this invention directs itself to a technique for highly localized measurements of complex microwave permittivity of materials.
More in particular, the present invention relates to a probe for non-destructive determination of complex permittivity of a material based on a balanced two-conductor transmission line resonator which provides confinement of a probing field within a sharply defined sampling volume of the material under study to yield a localized determination of the material""s complex permittivity.
One of the main goals of the near-field scanning microwave microscopy is to quantitatively measure a material""s complex microwave permittivity (dielectric constant and conductivity) with a high sensitivity of lateral and/or depth selectivity (i.e. to determine the material""s property over a small volume while ignoring the contribution of that volume""s surrounding environment). This is particularly important in measurements on complex structures, such as semiconductor devices or composite materials, where, for example, the permittivity of one line or layer must be determined without knowledge of the properties of the neighboring lines or underlying layers.
In microwave microscopy the basic measurement is a determination of the reflection of a microwave signal from a probe positioned in close proximity to a sample. Phase and amplitude of the reflected signal may be determined directly by using a vector network analyzer or by determination of the resonant frequency and quality factor of a resonator coupled to the probe.
In many cases, the phase of the reflected signal correlates to a large degree with the real part of the sample permittivity, whereas magnitude is dominated by the imaginary part of the permittivity (i.e., the microwave absorption of the sample). Measurements of the microwave transmission from the probe through the sample are also possible, however, such an arrangement generally does not yield a localized determination of a sample""s complex permittivity.
Many conventional approaches in microwave microscopy employ a coaxial probe geometry. An alternative to the rotationally-symmetric arrangement of the coaxial probes are planar structures such as a co-planar wave-guide or a strip-line wave-guide. Such an apparatus yields an imaging resolution on the order of the diameter or radius of curvature of the central conductor tip.
It is obvious, however, from considerations of classic electrodynamics that the volume of space over which such an apparatus determines the electrical properties of a sample is determined not by the local dimensions of the central conductor tip alone, but rather by a length scale given by the separation between the central conductor tip and the ground (outer) conductor or shield.
Therefore, in order to determine quantitatively the microwave properties of a material these properties must be devoid of non-uniformities on length scales at least a few times larger than the distance between the probe tip and the ground conductor while sufficient imaging contrast on length scales comparable to the radius of curvature of the tip can be easily achieved.
Furthermore, the inherent unbalanced character of the exposed part of the probe complicates any of the above-mentioned geometries due to the dipole-like current-flow in this area. The amount of radiation is critically dependent on the environment, i.e., the sample""s complex permittivity and the probe-to-sample distance, and thus affects the amplitude of the reflected signal (reflection measurement) or quality factor of the resonator (resonant technique). The result is a potentially erroneous determination of the sample""s microwave absorption.
It is, therefore, an object of the present invention to provide a technique for selective localized determination of a complex permittivity of a material.
It is another object of the present invention to provide a novel probe for the non-destructive determination of a sample""s complex permittivity based on a balanced two-conductor transmission line resonator which is symmetric with respect to an exchange of signal between the conductors that makes it possible to confine the probing field within the desired sampling volume which significantly reduces dependency of measurements on the sample volume""s environment.
It is a further object of the present invention to provide a measurement technique applicable in the frequency domain up to about 100 GHz in which the sample""s complex permittivity is determined with high accuracy either by a measurement of the phase and magnitude of a microwave or millimeter-wave signal reflected from the sample, as well as by a measurement of a resonant frequency and quality factor of a resonator formed by (or coupled to) a two-conductor transmission line, or by the capacitance measured between the two conductors of such a transmission line.
Furthermore, it is an object of the present invention to provide an apparatus for highly accurate determination of the complex permittivity of a sample which employs a probe capable of sharply localized measurements which can be easily controlled for modification of sampling volume as well as for the depth profiling.
In accordance with the principles thereof, the present invention is a novel probe for non-destructive measurements which includes a two-conductor transmission line comprising a pair of spatially separated, symmetrically arranged electrical conductors of circular, semi-circular, rectangular, or similar cross-section contour. One end of the transmission line (also referred to herein as the xe2x80x9cprobing endxe2x80x9d) is brought into close proximity to the sample to be measured and may be tapered (or sharpened) to an end having very small spatial extent. A signal is fed through the transmission line toward the sample, and a signal reflected from the sample is measured. For this purpose, the opposite end of the transmission line is connected to electronics for the determination of the reflected signal""s phase and magnitude. Measurements of the phase and magnitude of the reflected signal are broadband in frequency.
Preferably, for highly sensitive and accurate measurements, while employing less expensive electronics, a resonator is formed by a portion of the two conductor balanced transmission line with the conductors separated by air or another dielectric medium, and measurements of the resonant frequency and quality factor of the resonator are made. For example, such a dielectric medium may include a circulating fluid for temperature stabilization, or a high dielectric constant material for size reduction. In this type of embodiment, the opposite end of the transmission line is coupled to a terminating plate. Coupling to the resonator can then be provided by a conducting loop positioned close to the resonator. It is to be noted that an optional second coupling loop may be used for the measurement electronics.
Typically, the transmission line is operated in the odd mode, i.e., in a mode in which the current flow in one of the two conductors is opposite in direction to that in the other conductor.
The transmission line or the resonator may be partially enclosed by a metallic sheath. If a conducting sheath is used, the transmission line also supports an even mode, similar to that observed in a coaxial transmission line. When operated in the even mode, the interaction between the sample and the probe is similar to the coaxial symmetries.
When the probe is operated as a resonator, the two modes (odd and even) will in general result in two different resonant frequencies (due to dispersion), and can therefore be easily separated in the frequency domain to be powered and monitored independently. In order to enhance the dispersion, a piece of dielectric material is sandwiched between the conductors of the resonator.
The spacing between the two conductors of the resonator and their cross-sections must be properly chosen in order to maintain a resonator quality factor Q high enough for accurate measurements of the sample induced changes in the resonance frequency and the Q factor. For instance, the spacing has to be on the order of or greater than 1 mm, for Q greater than 1000 at 10 GHz.
When the resonator is enclosed in a cylindrical sheath formed of a high electrically conductive material, the sheath simultaneously eliminates radiation from the resonator and the effect of the probe""s environment on the resonator characteristics. At the same time, the sheath has an opening near the sample area, thus allowing an efficient coupling of the sample to the resonator. The upper part of the sheath makes an electrical contact with the terminating plate. The bottom part of the sheath may have a conical shape in order to provide physical and visual access to the sampling area.
The geometry of the cross-section of the probing end of the two conductor transmission line resonator determines the sampling volume, i.e., the spatial resolution of the measurement both laterally and in depth. Due to the symmetry of the near field electrical field distribution at the probing end, the subject novel probe allows for a determination of the in-plane anisotropy of the sample""s complex permittivity. In particular, measurements obtained with different probe orientations can be compared or subtracted each from the other.
In order to obtain a high spatial resolution, the diameter of the two conductors of the resonator at the probing end as well as their spacing are reduced in size to the smallest possible dimension by tapering each of the two conductors to a desired cross-section and gradually reducing their separation down to a value smaller than, or comparable to their diameter.
Alternatively, the portion of each conductor closest to the sample can be replaced with a scanning tunneling microscopy (STM) tip or with a metal coated optical fiber which may be tapered to a sharp point. Alternatively, an optical fiber may be used onto which two metallic strips have been deposited on opposite sides. In the case of the optic fiber having two metallic strips embedded therein, the optical fiber is tapered to a sharp point whereby the two metallic lines are gradually brought into close proximity to each other.
Alternatively, the entire transmission line resonator may be formed from a single piece of an optical fiber (or other dielectric bar with cylindrical or similar cross-section) with either a non-tapered or tapered probing end. In this structure, the fiber including the tapered portion has two metallic strips deposited on opposite sides thereof.
Additionally, the portion of the transmission line closest to the sample may be replaced with a multi-layer structure formed on a flat substrate by the subsequent deposition of the first conducting line, a dielectric spacer layer, and the second conducting line. Additionally, the portion of the transmission line closest to the sample may be replaced with a tapered slot line formed on a flat substrate.
Preferably, the spacing between the two probes can be adjusted by moving one of the two conductors with respect to the other by, for example, means of a piezoelectric actuator. Alternatively, the separation between the two probes may be adjusted by electrostatic, magnetic, or other means. Variations of the separation between the two conductors results in a change in the distribution of the electric fields near the probing end, and thus, a modification of sampling volume. In this approach measurement of depth profiling may be accomplished.
In embodiments where a metal coated optical fiber is used, the desired separation between the fibers at the probing end can be monitored by means of measuring the amplitude and/or phase of an optical signal transmitted from one fiber to the other.
The distance between the probe and the sample can be controlled by tracking the microwave response, or by controlling a tunneling current between the probe and the conducting sample. Also, any other distance measuring mechanism known in the field of near field scanning optical microscopy can be employed. Such techniques include detection of shear force, tuning fork oscillators, and reflection at the sample surface of an optical signal originating in the optical fiber.
These and other novel features and advantages of this invention will be fully understood from the following detailed description of the accompanying Drawings.